1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus comprising the electrophotographic photosensitive member.
2. Related Background Art
The electrophotographic process comprises, for example, forming a latent image by charging and exposing a photosensitive member having at least a photosensitive layer on a conductive support; forming a developed image with the aid of a toner; transferring the developed image to a transfer medium that is mainly paper; and removing/recovering (cleaning) the transfer remaining toner. The electrophotographic photosensitive member used in this case is required to have a necessary sensitivity, electric characteristics and optical characteristics in conformity with the applied electrophotographic process. Electrical and mechanical external forces, including charging, toner development, transfer to paper and cleaning of remaining toner, are exerted directly to the surface of the photosensitive member that is used repeatedly, and hence the surface concerned is required to have the resistance to these external forces. More specifically, resistance is required against the scratch and abrasion due to sliding friction, and particularly, resistance is required against the chemical deterioration due to ozone and NOx generated at high humidities when a charging scheme involving discharge is adopted. Furthermore, at the time of repeating the cleaning of the remaining toner, there occurs a problem that the toner adheres to the photosensitive member surface and cleaning with a blade causes scraping of the photosensitive member surface by the blade, and hence the photosensitive member surface is required to have such characteristics as slidability, releasability, and antistaining property.
In order to meet the demands described above, as the material for the surface layer of the photosensitive member, it is widely proposed to use a resin excellent in releasability and slidablity such as represented by fluorine-based resins, and a highly hard resin material such as represented by silicone resin, urethane resin, and unsaturated ester materials.
However, a material fulfilling the above described various characteristics has not so far been found. More specifically, a fluorine-based resin alone is low in hardness so it can hardly suppress the scratch generation, and it is scarcely soluble in common solvents so formation of its film is not easy.
On the other hand, there is reported an example in which for the photosensitive member, used is highly hard materials such as a cured silicone resin taking advantage of the high reactivity of alkoxysilane; however, these resins are not satisfactory in such aspects as slidability, electric characteristics at high humidities, and releasability. Furthermore, these cured materials are highly reactive to the hydroxy group so that there are some constraints on the solvent for use in photosensitive layer coating, and additionally the curing reaction slowly proceeds in these materials under the effect of the contained moisture so that the stability of the coating solution is poor, which is problematic from the viewpoint of the productivity of the photosensitive member.
Additionally, a material for cured film formation through cleavage of unsaturated bonds such as prepolymers of diallylphthalate resin is generally of the radical polymerization type, and the coating solution using this material is comparatively stable to moisture; however, this material can merely yield a cured material unstable in electric characteristics including insulation resistance owing to the poor curing on the film surface caused by the polymerization inhibition effect due to the oxygen in the air, and owing to the carbon-carbon bond cleavage reaction and the like caused by light irradiation when a photoinitiator is used. Accordingly, there have been problems that the transfer efficiency is degraded due to the elevated surface free energy and the image blurring is caused by moisture absorption.
On the other hand, the material for use in the surface layer of the photosensitive member is required to have the electric characteristic that secures smooth charge transfer even in the interior of the uppermost surface layer, in addition to the above described hardness and slidability. In this connection, if the surface layer has no function for conducting charge transfer, the charge accumulation occurs in the interior of the photosensitive layer in such a way that the repetition of the electrophotography process of charging-exposing results in the elevated residual potential, leading to the image quality degradation.
For the purpose of solving this problem, a method has been proposed in which a charge transport material is contained in the surface layer. For example, when curing is made by adding a charge transport material to alkoxysilanes, the charge transport material and the siloxane component are frequently poor in compatibility with each other, and when a charge transport material is added to a resin having a high polarity unit such as urethane resin, the charge mobility due to the charge transport material is lowered, actually leading to unsatisfactory electrophotographic characteristics.
Furthermore, among the thermosetting resins, various materials are not compatible with mere application of heat treatment, but need the addition of curing catalysts such as curing accelerators and polymerization initiators. However, when such a curing catalyst remains in the cured film, possibly there occur such an adverse effect that the charge transfer is inhibited by even a small amount thereof, or the electric resistance of the cured film is degraded. On the other hand, a coating material added with a curing catalyst tends to undergo slowly processing reaction even at ambient temperature, resulting in degradation of the stability of the coating material, which leads to an adverse effect that the mass production of coating materials and the storage thereof become a hard task.
Additionally, main schemes for the electrophotographic photosensitive member involve electric discharge, among which a charging scheme, involving discharge in a thin gap between the electrophotographic photosensitive member and the charging member applied both with a DC voltage and with an AC voltage, is the one excellent in charging stability among the contact charging schemes, but involves a phenomenon that the surface composition of the electrophotographic photosensitive member is destroyed in an oxidatively deteriorated manner, leading to an elevation of the surface free energy causing the transfer efficiency degradation. Furthermore, when a thermosetting resin is used, the abrasion amount of the photosensitive layer is small, and hence the destroyed material in an oxidatively deteriorated manner possibly causes a problem of image blurring due to moisture absorption.
Additionally, in general, when the surface layer comprising a cured resin is provided on the photosensitive layer comprising a thermosetting resin, if, as is the case for the surface layer comprising a fluorine-based resin as the cured resin, the cured resin is totally different from the photosetting resin in chemical composition, the adhesiveness of the surface layer to the photosensitive layer is poor so that a part of the photosensitive layer may sometimes be peeled when used over a long period of time in the electrophotographic process, resulting in an adverse effect of generating deficient images.
Additionally, in general, the more the crosslinking density of cured resins is increased, the harder the cured resins are surely, but concurrently the more brittle the cured resin is. Furthermore, such curing resins lead to extreme increase in the surface roughness of the electrophotographic photosensitive member in the long term course of the use thereof, sometimes causing adverse effects on the images, and accordingly the fact is such that no satisfactory materials have been obtained in adaptation of curing resins to the surface layer.
Additionally, Japanese Patent Application Laid-Open No. H10-228126 discloses an example in which the surface layers of the photosensitive members are made to contain a charge transport material containing phenolic hydroxy groups and hydroxyalkyl groups; however, even these photosensitive members cannot meet the recent demands for high durability, high productivity and high image quality, and the fact is such that all the items including the mechanical strength, residual potential, productivity and the like are not yet sufficiently satisfied.
Additionally, Japanese Patent Application Laid-Open No. 2002-82466 discloses that an electrophotographic photosensitive member, which comprises a protective layer containing a resol type phenolic resin and a metal particle or a metal oxide particle, exhibits little elevation of the residual potential in the low humidity environment, provides such high quality images as free from blurring and smearing in the high temperature environment, displays excellent releasability, and has excellent durability against the generation of abrasion and scratches.